1. Field of the Invention
This invention relates to a process variable transmitter, for example such as a two-wire differential-pressure transmitter, and more particularly to a process variable transmitter capable of correcting a process variable to be measured, with reduced electric power consumption.
2. Description of the Prior Art
There are known a variety of process variable transmitters. One such conventional process variable transmitter is a differential-pressure transmitter for detecting a signal indicative of a differential pressure as a process variable to be detected and for transmitting such a signal to a certain location. The differential-pressure signal produced by a differential-pressure sensor is influenced by unmeasured secondary process variables such as temperature and static pressure, and hence is relatively unreliable for the standpoint of accuracy. Transmitters used in laboratories are designed to take the above influences into account. Thus, unmeasured secondary process variables such as temperature and static pressure are detected and then a differential-pressure signal is corrected on the basis of such detected process variables for gaining an accurate measured value.
More specifically, a prior process variable transmitting arrangement includes a differential-pressure transmitter installed on a pressure pipe for detecting a process variable to be measured, and temperature and pressure transmitters mounted on the pressure pipe. Process-variable signals from these transmitters are delivered to a receiving instrument, in which the output signal from the differential-pressure transmitter is corrected.
Where such a correcting means is incorporated in a two-wire transmitter, the following difficulties arise: two-wire transmitters are normally required to operate at 24 V with an electric current ranging from 4 to 20 mA. When the transmitter is coupled with a receiving instrument, the transmitter should operate with an electric power of about 50 mW or at 12 V with 4 mA in reality. With such a power requirement, the transmitters for laboratory use consume a moderate amount of power where the above correcting means is incorporated. However, those transmitters which are for sale on the market have many transmission wires, and require increased power consumption, so that they cannot find widespread use in actual applications. The known transmitting arrangement is relatively costly as it needs to be equipped with the differential-pressure transmitter, temperature transmitter and pressure transmitter. Correction in the receiving instrument is liable to be less accurate because of an accumulation of errors arising in the transmission via the transmission wires to the receiving instrument. The less accurate correction fails to achieve desired linearity between an actual differential pressure and a measured differential pressure, with the consequence that the differential pressure as measured will contain unwanted errors.